Zoom lens system for use in a microfilm projection apparatus

Abstract

A zoom lens system comprising, in order from an enlargement side, a first lens group of a negative power, a second lens group of a positive power and a third lens group of a positive power. The first lens group is fixed during a zooming operation from a longest focal length condition to a shortest local length condition and includes, in order from the enlargement side, a positive lens element, a negative lens element and an aperture stop. The second lens group includes two positive lens units which are movable independently of each other toward a reduction side during the zooming operation. The third lens group is also movable toward the reduction side during the zooming operation and includes, in order from the enlargement side, a positive lens element and a negative lens element.

What is claimed is: 
     
       1. A zoom lens system comprising, in order from an enlargement side, a first lens group of a negative power which is fixed during a zooming operation and includes, in order from the enlargement side, a positive lens element and a negative lens element,   a second lens group of a positive power which is movable during a zooming operation and includes, in order from the enlargement side, a first positive lens unit and a second positive lens unit, said first positive lens unit having a positive lens element and a negative meniscus lens element, said second positive lens unit having two positive lens elements and two negative lens elements, said first and second positive lens units being independently movable toward a reduction side during a zooming operation from a longest focal length condition to a shortest focal length condition, and   a third lens group of a positive power which is movable during a zooming operation and includes, in order from the enlargement side, a positive lens element and a negative lens element.   
     
     
       2. A zoom lens system as claimed in claim 1, wherein said first lens group further includes an aperture stop. 
     
     
       3. A zoom lens system as claimed in claim 1, wherein the positive lens element and the negative lens element of said first lens group are cemented together so as to form a compound lens unit. 
     
     
       4. A zoom lens system as claimed in claim 1, wherein said second positive lens unit includes, in order from the enlargement side, a positive lens element, a biconvex lens element, a negative lens element, a biconvex lens element and a negative lens element. 
     
     
       5. A zoom lens system as claimed in claim 1, wherein said positive lens element of the third lens group has a convex surface directed to the reduction side. 
     
     
       6. A zoom lens system as claimed in claim 1, satisfying the following condition (a) and (b):   0.35<f2L/f1<0.45                                           (a)       -0.25<e1/f1<0                                              (b)     where f2L represents the focal length of said second lens group in the longest focal length condition, f1 represents the focal length of the system in the longest focal length condition, and e1 represents a calculated distance between a principal plane of the first lens group and a principal plane of the second lens group in the longest focal length condition.